GLACIAL DRIFT. l8l 



Fig. 50. This represents an area of about five rods square on the south- 

 west side of Mt. Monadnock. Here they occur near the summit of the 

 mountain, not along the sides of a valley. 



There are many examples of embossment in the state as good as these. I have 

 noted a few of them : The top of Mt. Kearsarge ; about the Lake of the Clouds, be- 

 tween Mts. Washington and Monroe, though the action of the frost is injuring their 

 perfection ; along the Pemigewasset valley in Woodstock ; Baker's River valley, in the 

 west part of Plymouth and the east part of Rumney, — also higher up, less perfectly ; 

 along the Northern Railroad, between Orange summit and Grafton Centre ; Sanborn- 

 ton, west of Cawley's pond. The Huronian and Cambrian slates of the Connecticut 

 valley abound in fine examples of these rock domes. Others are specified in the 

 column of remarks about stri^, further along. The narrow belt of rock in which the 

 principal mica quarries are located, between Groton and Keene, is noticeable for the 

 very fine embossment of the ledges all over its course. Granitic and calcareous rocks 

 decompose readily ; but the general form of the embossment will remain after the 

 markings have been obliterated, so that a practical eye will recognize the fact of their 

 glaciation. 



Striatiox. 



By far the most important effect of the ice-movement is the striation. 

 All these domes and the worn sides of ledges exhibit lines more or less 

 distinct, or passing into grooves which have been produced by hard rock 

 fragments frozen into the bottom of the ice-sheet. As the mass moved 

 along, these fragments acted as chisels or gouges, deeply scratching 

 the ledges. It is obvious that the grooving instrument must have been 

 harder than the rock affected ; hence pieces of soft rocks, like limestone, 

 would not leave any mark of their passage. These markings are often 

 obliterated by disintegration, and it is difficult to find their direction. 

 Several expedients may be resorted to: First, search for veins or pro- 

 jections of tough materials, upon which faint lines may possibly be 

 found. These harder substances show the action, because they have 

 successfully resisted disintegration. Again : it may be necessary to wet 

 these projections, or the whole surface of a smoothed rock, in order to 

 discover the direction of the striae. Sometimes deep grooves may be 

 found, which will indicate the course as well as scratches. 



These markings vary from the finest scratches visible to deep furrows. 

 They may be straight or curved, the latter where the abrading chip 



